When given a choice, the Swainson’s Thrush and other migrating bird species will pick native plants to feed on over non-native plants. Photo: Josh Haas.

By Howard Meyerson

Kay Charter isn’t shy about letting people know that native plants are preferred when it comes to providing nutrition for birds. She and her husband, Jim, restored 30 acres of prairie on their 44-acre Omena bird preserve, Charter Sanctuary. The couple created it to provide a landscape where bird conservation was a top priority.

“The most important thing is removing non-native plants,” notes Charter, executive director for Saving Birds Thru Habitat, a conservation and education nonprofit that promotes protecting, enhancing and restoring habitat for birds. “You don’t have to buy a bunch of plants. We did that here, though, spent thousands of [grant] dollars on native plants—and converted 30 acres to prairie by planting native grass and forbs [flowering plants] . . . and planted shrubs like elderberry and service berry around a wetland, and other trees and shrubs.”

Native plant species have larger insect populations, Charter said. Insects are crucial because they provide the protein young birds need to grow. Charter recognized that fact some years ago, and she ardently promotes Dr. Doug Tallamy’s work.

Common elderberry is among the native fruiting plants that migrating birds will choose to eat in fall.

Biodiversity

Tallamy is a strong proponent for biodiversity. He is professor and chair of entomology and wildlife ecology at the University of Delaware, and author of Bringing Nature Home. He is known for his research into bug life and abundance in native and non-native plant habitats.

Insects and native plants evolve together, Tallamy says. They adapt to the chemicals in plants that plants use to protect themselves. The result is caterpillars and other insects can consume those plants and thrive without harm, while invasive plants remain inedible and have fewer insects and less food for birds. Tallamy writes:

“We used to think this was good. Kill all insects before they eat our plants! But an insect that cannot eat part of a leaf cannot fulfill its role in the food web. We have planted Kousa dogwood, a species from China that supports no insect herbivores, instead of our native flowering dogwood (Cornus florida) that supports 117 species of moths and butterflies alone. In hundreds of thousands of acres we have planted goldenraintree from China instead of one of our beautiful oaks and lost the chance to grow 532 species of caterpillars, all of them nutritious bird food.

“My research has shown that alien ornamentals support 29 times less biodiversity than do native ornamentals. Our studies have shown that even modest increases in the native plant cover on suburban properties significantly increase the number and species of breeding birds, including birds of conservation concern.”

More songbirds

Though not a panacea, Charter’s restoration work at the sanctuary has resulted in some success. Nesting birds are more abundant, but species like the Bobolink and Meadowlark continue to decline.

“Last year we made a list (of nesting birds) and found more than 150 pairs comprised of 44 different species,” Charter said. “When I first came here we didn’t have a whole lot of nesting birds. The prairie has not affected the upland species as we hoped. The grassland species are still declining.

“We’re working on a management plan for the sanctuary and have decided to force more rapid succession by planting shrubs and trees. Early succession forests are used by so many birds. Mostly we focus on songbirds here and they don’t eat plants. They eat insects. A few will eat seeds and berries at certain times, but the big food source is insects.”

Clear preferences

Dr. Jen Owen, associate professor at Michigan State University’s Department of Fisheries and Wildlife and Large Animal Clinical Sciences, says birds prefer native plant species when they have a choice. Those were the findings of a 2012 study that examined the food preferences of migrating birds.

The two-year research project, conducted by Yushi Ouchi, a graduate student in her program, examined what birds ate and how healthy they were. They were netted on two adjacent tracts of land in Rose Lake Wildlife Research Area. One was dominated by native plants such as spicebush, common winterberry, American highbush cranberry, and common elderberry. The other was dominated by exotic plants such as autumn olive, Japanese honeysuckle, European buckthorn, and multifloral rose. Besides tallying where the birds were netted and conducting fecal studies to learn just what they ate, a number of Swainson’s Thrushes and Catbirds were fitted with radio-tags and tracked to see where they spent time.

“Fruit is incredibly abundant in fall so a lot of birds may switch to eating primarily fruit for energetic benefits,” Owen said. “They don’t have to expend a lot of energy to [find] it. Studies have found that invasive plant fruits are high in sugar but not high in lipids [fat], and [migrating] birds are trying to fatten up. Some will double their body mass for migration. What we found is overwhelmingly, no matter which habitat they were caught in, they spend most of their time in native habitat. By collecting fecal samples we could identify what they were eating. The most common fruit consumed was the common spicebush.”

Why they prefer spicebush is a question yet unanswered. What is known is that spicebush is high in lipid content, “a very efficient fruit to be eating,” Owen said. Exotic plants have lower lipid content but can have more antioxidants.

Health effects

Other study findings include that Catbirds foraging in native plant terrain had better immune functions. They were also less stressed. Owen said that wasn’t so with Swainson’s Thrushes. Their results trended in the same direction but were not statistically significant.

Study birds also ate disproportionately low amounts of autumn olive. The exotic plant was only 5 percent of their diet though it was available on 65 percent of the habitat. Buckthorn was 30 percent of the diet and made up 20 percent of the non-native habitat. Where there was choice, the birds ate buckthorn.

In native plant terrain where spicebush was 25 percent of the habitat, it was 65 percent of the diet for catbirds and Swanson’s Thrushes.

“It is the quality of the fruit that drives the selection,” Owen said.